This invention relates to a high-purity silicone ladder polymer having lower alkyl groups in side chains, as well as a process for producing said polymer. More particularly, this invention relates to silicone ladder polymer that can advantageously be used as a material for making protective films, inter-level insulation films, etc. in semiconductor and other electronic devices. The invention also relates to a process for producing such high-purity silicone ladder polymer.
Because of their unique molecular structure, silicone ladder polymers excel in heat resistance, electrical insulating properties, chemical resistance and anti-aging property and they have conventionally been used as materials for making protective films and inter-level insulation films in electronic parts or semiconductor devices, etc.
Conventional processes for producing such silicone ladder polymers are described in Reference 1 (Japanese Patent Public Disclosure No. Hei 3-20331) and Reference 2 (Japanese Patent Public Disclosure No. Hei 2-107638). Silicone ladder polymers have a structure represented by the following formula and are characterized by ladder bonds: ##STR1##
The process described in Reference 1 for producing the silicone ladder polymer of interest starts with reacting an organotriacetoxysilane with an equivalent amount of alcohol or water in an organic solvent to prepare an alkoxyacetoxysilane. Then, the resulting alkoxyacetoxysilane is polycondensed in an organic solvent in the presence of sodium hydrogencarbonate to give a prepolymer. Subsequently, the prepolymer is thermally polycondensed in the presence of a catalyst such as an alkali metal oxide or an alkaline earth metal hydroxide or a fluoride thereof or triethylamine to yield a polyorganosilsesoxane which is a silicone "ladder" polymer resin.
The process described in Reference 2 comprises dispersing a catalyst in a prepolymer of ladder-type organopolysiloxane and polycondensing the dispersion by uniform heating under exposure to microwaves, whereby the intended organopolysiloxane of a ladder type is produced. The catalyst used in this process is selected from among alkali metal hydroxides and metal fluorides.
The silicone ladder polymers produced by these conventional methods have had the problem that they contain large amounts of impurities or by-products. This is because the conditions for preparing the prepolymer are not controlled in such a way as to produce a ladder-type polymer and, hence, by-products such as linear or cyclic condensation polymers are prone to occur, increasing the chance of gelation to take place when the prepolymer is polymerized to a polymer of a higher molecular weight. This is why the silicone ladder polymers produced by the methods of References 1 and 2 contain large amounts of impurities and by-products.
If the method of polymerization described in Reference 1 is conducted at high concentration, the chance of the formation of the by-products exemplified above increases and they will remain in large amounts in the silicone ladder polymer produced. Such residual by-products are very difficult to remove.
To implement the method described in Reference 2, the catalyst must be dispersed uniformly in the prepolymer but this results in using the catalyst in an increased amount. The catalyst is an impurity to the silicone ladder polymer to be produced and if the method described in Reference 2 is employed, the silicone ladder polymer will be produced under such conditions that the catalyst is difficult to remove. Hence, the silicone ladder polymer produced by the method of Reference 2 contains a large amount of impurity catalyst, which is also very difficult to remove.
Another problem with the prior art methods is that it is difficult to produce silicone ladder polymers of higher molecular weight. In order to increase the molecular weight of the silicone ladder polymers to be produced by the conventional methods, polycondensation of the prepolymer must be carried out at high concentration but this increases the chance of its gelation for the reason already mentioned above. Hence, the maximum weight average molecular weight that could be attained was only on the order of 10.sup.4 and it has been impossible to produce silicone ladder polymers having weight average molecular weights in excess of 100,000. Furthermore, the molecular weight distribution (weight average molecular weight divided by number average molecular weight) of the silicone ladder polymers that are produced by the conventional methods is so wide (100 or more) that if one attempts to form a predetermined pattern in a film of the polymer using a solvent, the cross-sectional profile of the pattern will deteriorate. Hence, it has been difficult to form this films from the silicone ladder polymers produced by the conventional methods.